Why does our working society expect a tip?

ok so this ticks me off i'm a full-time college student and often find myself on a tight budget to me $5 could mean a days worth of food. ok so not to be stingy or anything but why should i tip my hairdresser an extra $5 WHEN SHE charges $25 for a haircut...and why is it that some people expect you to tip them when they are totally rude etc. a waitress. at a bar or club you pay minimum $5 for a drink plus expected to pay an extra $1 ok that sometimes bums me out when i go out and sometimes i'm kept from doing certain things . so now why is it that our society has accustomed to such thing "tipping" or expect for one to tip I love to tip when i have been treated great but sometimes these people are rude...not all of them but most of them.